border-1px($color) // 1像素边框
  position:relative
  &::after
    display:block
    position:absolute
    left:0
    bottom:0
    width:100%
    border-top:1px solid $color
    content: ''

border-none()
  &:after
    display:none

bg-image($url) // 几倍背景图插入
  background-image: url($url+"@2x.png")
  @media (-webkit-min-device-pixel-ratio: 3),(min-device-pixel-ratio: 3)
    background-image: url($url+"@3x.png")

    